Object is mirrored at incorrect location
The object that you have set as the Mirror Object ('Cylinder') on the object you want to mirror ('Cylinder.010') is not at the centre of your tank, therefore, neither is the mirroring.

If you still want to use a Mirror Object then choose a different object to use as your Mirror Object (one that has it's origin at the centre of the tank).

For example, here I change the Mirror Object to be 'Cube.004' and the mirroring is correct: